Table of Contents

Sound is the perception of vibrations and the air within the range of human hearing. It is created by the vibration of objects and surfaces which emit pressure waves. These consist of the original vibration along with its’ harmonics; usually caused by part of the sound source or its close surroundings.

Frequency

Frequency is measured in Hz. This stands for Hertz, which means the number of cycles per second. The human ears are able to detect frequencies between 20 and 20000Hz, although the upper range tends to become reduced with both age and hearing damage.

Frequency is closely related to Pitch, which is a psychoacoustic effect where the brain perceives frequencies as musical tones and high to low scales. The wavelength is the distance over which a waveform repeats itself. The higher a given frequency, the shorter its’ wavelength will be.

Physical Properties of Sound

Sound travels as a sequence of pressure waves that travel through liquids, solids and gases. They can be reflected, refracted or attenuated by the material it is traveling through. Generally the denser a material, the faster the wave will propagate but its’ energy is absorbed much quicker.

Acoustics is the science of physical waves and their interactions with their environment. It helps us understand how sounds and their reflections and refractions are heard, perceived and are affected by our environment. Some environments are designed specifically to provide a “clean” listening or recording experience without reflections and reverberation.

Hearing and Our Ears

The vibrations in the air are funneled and reflected by the pinna into the ear canal. The sound travels down the ear canal and is amplified as it travels into a smaller area. It then hits the Tympanic Membrane (or eardrum), which causes it to vibrate. The eardrum transfers these vibrations to a small bone called the Maleus which transfers the vibrations in turn to the Incus and Stapes bones. The Stapes vibrates against the Oval Window to transfer the vibrations to the fluid called Endolymph inside the Cochlea. The Cochlea is spiral shaped, starting wide and tapering down and has 3 sections. It is filled with tiny hairs called Cilia that sense movements in the Endolymph. As the moving fluid stimulates the Cilia, they translate the moment into nerve impulses for the brain to perceive as sound. Longer hairs at the beginning of the Cochlea pick up the lower frequencies and this continues to the higher frequencies, which will be picked up by the smaller Cilia at the furthest end. Electrical signals are then sent from the spiral ganglion down the cochlear nerve to the auditory cortex, the area of the brain responsible for processing sound information.

Our ears are delicate and often irreparable limits to the volume or sound pressure levels they can be exposed to. This is mostly due to the mechanical limits and weaknesses/delicacies of the moving parts. The bones in the ears can only vibrate so much and can damage the cartilage holding them in place. The tympanic membrane can be ripped or punctured by loud pressure spikes and sustained. The Cilia in the Cochlea can become broken and reduce the overall sensitivity to that particular area of the frequency range.

Pressure Levels and Volume

Sound Pressure Level (SPL) is a measurement of sound pressure that is logarithmic and is measured in decibels (dB). It is defined as the number of decibels above a “normal” level, often 20 µPa RMS at 1kw. A 3dB increase represents around a doubling of pressure levels and a 10dB increase represents an increase by a factor of 10. A rustle of leaves or a whisper is around 20dB. A normal conversation is normally 30-40dB and a performance at a theatre around 50dB. Industrial noise such as trains or heavy machinery normally ranges from 60-80dB. Rock and dance music events often exceed 120dB which can be damaging to hearing especially if hearing protection is not used. The threshold of pain in hearing is around 130dB although it may be higher in those who have been exposed to high levels of sound beforehand.